Drug resistance is a major obstacle of most of anticancer therapeutics. Several clinical studies have described reduced response to antiangiogenic therapy targeting VEGF pathway over time followed by regrowth of treated tumors. As tumor angiogenesis is governed by multiple pathways, one difficulty in antiangiogenic therapy is the selective up-regulation of other pro-angiogenic factors, including platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), and hepatocyte growth factor (HGF), which bypasses VEGF and renders tumors refractory to anti-VEGF therapy. Although targeting VEGF pathway carries an enormous therapeutic potential, it is very likely that resistance to inhibition of these pathways will emerge as a potential obstacle to be overcome in clinical practice. The main goal of this study is to elucidate potential cellular and molecular mechanisms mediating tumor resistance to anti-VEGF therapy. The main hypothesis is that induction of other proangiogenic signaling, independent of VEGF, is the leading mechanism of development of tumor resistance and is associated with hypoxia in tumor microenvironment. Application of the innovative approaches to study this significant relationship could greatly expand our limited understanding of bypass mechanisms of tumors to anti-VEGF agents and provide rationale to select multi-targeting agents that counteract such mechanisms of resistance mediated by alternative proangiogenic signaling. This goal will be accomplished through the following Specific Aims: 1) Identify the pathways involved in crosstalk between VEGF and VEGF-independent proangiogenic signaling transduction in stroma-tumor co-cultures; 2) Evaluate the roles of stromal cells rescuing tumors from the anti-VEGF therapy-induced hypoxia; and 3) Identify in vivo resistance signatures to anti-VEGF treatments using xenograft model. From these experiments, we will identify potential pathways of resistance to be targeted, and to validate in vitro and in vivo pre-clinical models for identifying resistance to targeted agents, thereby proving insight into the design of future therapeutic strategies for tumors refractory to anti-VEGF therapy.